The only espionage trial in the U.S. that has resulted from the Vietnam war opened yesterday with testimony from a paid informant for the CIA and FBI that she delivered packages given her by Vietnamese expatriate David Truong to communist officials who called Truong " one of our people in Washington."

The testimony by informant Dung Krall came following jury selection in U.S. District Court in Alexandria in the case of Truong and United States Information Agency employe Ronald L. Humphrey. Truong and Humphrey are charged with funneling classified diplomatic documents to the Hanoi government through an international espionage network.

Those documents ere characterized yesterday to Truong's defense attorney, Michael E. Tigar, as "diplomatic chit chat." In his opening arguments, Tigar asserted that Truong "is nobody's agent and nobody's spy."

Humphrey's attorney, Warren L. Miller, said his client's story "is the case of a man who loved too much and trusted too much." He said Humphrey loved his common-law wife, Kim, so much that he delivered what he thought were innocuous diplomatic cables to Truong in an effort to secure her release from a Vietnamese prison camp.

Humphrey is accused of taking the documents from his office at USIA and giving them to Truong, who is accused of forwarding them through couriers to Hanoi officials in Paris.

Krall, the government's first witness, testified about her dealings with Truong and Humphrey and also linked former Vietnamese U.N. Ambassador Dinh Ba Thi to the case. The same grand jury that indicted Truong and Humphrey named Thi an unindicted coconspirator, and he was recalled by his government last February after the U.S. government, in an unprecedented move, ordered him from this country.

A one point, Krall testified, Truong told her that Thi "doesn't know not to speak discreetly on the telephone. Mr. This doesn't know that the United States is a hostile country to Vietnam."

"You mean Mr. Thi asked you to send the documents over the telephone?" Krall testified she asked Truong.

"'Yes, he did,'" Krall testified Truong replied. "'I already explained to him and I'm sure it won't happen again on the telephone.'"

Krall, who said she was paid $1,200 a month by the CIA and FBI, testified she delivered to Thi a package given her by Truong, and on that occasion, in February 1977, she asked the ambassador if there was anything she could do to help his government.

Thi's reply, according to Krall, was that "what my father did was enough because my father was a communist." Her father had been a Vietcong ambassador to the Soviet Union.

Telling Thi she wanted a visa to visit her family in Vietnam. Krall testified, she asked the ambassador if her husband, a U.S. Naval intelligence officer, would have to steal documents for Vietnam. The reply, Krall said, was that "Mr. Truong would do that. He said Truong's different. He volunteered to do that."

She said she then asked Thi if he had any message to give Truong when she returned to Washington. "Tell him to send the same things he's been sending," Krall quoted the ambassador as saying.

On at least two occasions, Krall testified, she delivered packages from Truong to Vietnamese officials in Paris. These packages, she testified, contained letters, pictures of Vietnamest refugees and books.

She also described a conversation she said she had with Truong in an unspecified DUPont Circle cafeteria on April 19, 1977. Truong, Krall testified, told her he had a friend "two stole a pass to the State Department. He can walk up to the seventh floor and get anything he wants." Krall continued. "I said that must be very helpful to the Vietnam government and Mr. Truong was smiling."

Krall is expected to continue her testimony as the trial continues today.

In his opening argument, Tigar, Truong's attorney, said he plans to call as witnesses "employes of the U.S. department of State who'll evaluate" the cables his client is accused of funneling to the Hanoi government."They'll tell you what they are," he declared.